In recent years, various integrated circuit technologies have been proposed that use organic semiconductor thin-film transistors. It is expected that it will be possible to fabricate such integrated circuits by a simple, convenient manufacturing method, such as printing technology or the like, while other advantages include the possibility of realizing large-area processing and low-cost manufacturing processes, and good compatibility for fabrication on flexible substrates. This is also attracting expectations for the good compatibility with integrated circuit technology that will enable the low-price supply of electronic equipment, such as portable displays and electronic tags, such as electronic price tags and electronic labels and the like.
The development of thin-film transistors using organic semiconductors has been gradually becoming more active since the latter half of the 1980s, and in recent years has reached the point where, in terms of basic performance characteristics, it has surpassed amorphous silicon thin-film transistors. A representative example of this is the performance of a thin-film transistor using pentacene as the organic semiconductor, reported by Schön, et al., in Science, volume 287, page 1022 (2000).
The basic device structure of an organic semiconductor thin film up to now has generally been either what is called a bottom contact structure, as shown in FIG. 21, comprising layers constituted by a gate electrode 21 formed on a substrate 11, an insulation layer 31 formed on the gate electrode 21, on which are then formed, in parallel and simultaneously, a source 61 and a drain 41, and a semiconductor layer 51 formed thereon; or what is called a top contact structure, as shown in FIG. 22, comprising layers constituted by a gate electrode 22 formed on a substrate 12, an insulation layer 32 formed on the gate electrode 22, a semiconductor layer 52 formed on the insulation layer, on which are then formed, in parallel and simultaneously, a source 62 and a drain 42. These device structures are both characterized by the source and drain being fabricated in parallel and simultaneously. In endeavoring to improve basic properties, such as the high-speed responsiveness and low-voltage driving of a thin-film transistor, one of the most critically important factors is to shorten the distance between the source and drain, that is, the channel length. However, if the type of structure shown in FIG. 21 or FIG. 22 is used, just how short a distance between the source and the drain can be obtained depends in most cases on the basic fine processing technology. Although various techniques have been studied up until now, the problem is that a superior technique has yet to be established.
Of the known methods of fabricating the source and drain in parallel and simultaneously, one of the most simple and convenient methods is to adapt a method of forming electrodes under a vacuum, such as by vacuum deposition or sputtering or the like, using a mask. A problem with this method is that, so long as an ordinary mask is used, it is difficult to obtain a short source-drain distance of or below 10 μm. Another problem is that when a special mask is used that can be adapted to fine processing a single fabrication of an electrode clogs the mask, making it unsuitable for mass-fabrication.
The most representative technology used to obtain a shorter channel is photolithography. With respect also to organic thin-film transistors, there are reports, including Applied Physics Letters, volume 76, page 1941, 2000, that describe the use of photolithography technology to form channels having a shorter length, thereby obtaining superior thin-film transistor characteristics. However, in order to thus apply the technology, the semiconductor layers of the thin-film transistor are composed of organic material, which makes it difficult to introduce the operation of washing off the photo-mask with an organic solvent. Moreover, even if manufacturing processes were to be devised to which photolithography could be adapted, thereby enabling the performance provided by a thin-film transistor to be utilized, when the cost and time requirements of the photolithography technology are taken into consideration, it becomes impossible to achieve the low-cost, low-energy production that characterizes the use of organic material for thin-film transistors.
It is known that electron beam lithography technology can be used to obtain a shorter channel length than that obtained using photolithography technology. There is a report that describes a technology that achieves a very short channel length of 30 nm by using electron beam lithography, thereby enabling low-voltage driving of 0.35 V/decade at a source-drain voltage of 1 V (Applied Physics Letters, volume 76, page 1941, 2000). However, a problem with this technology is that it requires the adapting of electron beam lithography, which, being an extremely high-level, costly technology, means that the characterizing ability to adapt a simple, convenient process, such as printing or the like, provided by the use of organic material for thin-film transistors, is not achieved. Another problem is that of reduced throughput.
A technology that uses printing to fabricate devices is reported in Science, volume 290, page 2123, (2000). This achieves a short channel length of 5 μm by positioning fine spacer rods at the electrodes between the source and the drain. However, a problem with the technology is that the channel length depends on the width of the spacers, which necessarily means a reliance on fine-processing technology for how fine the spacers can be manufactured.
The use of a static induction transistor that is a transistor, in which the control of the channel length is not dependent on fine processing technology, has been reported (Nature, volume 372, page 344, 1994, and Synthetic Metals, volume 111, page 11, 2000). With this transistor structure, the channel length can be controlled by the thickness of the films that are fabricated. However, the thin-film transistor in this case has a different operating principle, so the circuit design pointers have to be changed in order to incorporate it as a conventionally applied thin-film transistor. Another problem is that although source-drain fabrication is easy, it is extremely difficult to fabricate the gate electrode.
As described above, in order to improve transistor characteristics, it is necessary to shorten the channel (the distance between the source and the drain) in which the current flows. In the prior art, the channel has been shortened adapting a high-level, fine processing technology, such as photolithography or electron beam lithography. It has been difficult to fabricate low-cost, high performance devices with these methods, due to the very high level and high cost of the adapted fine processing technologies.
Furthermore, drastically reducing the channel length has given rise to a problem in terms of the transistor characteristics, caused by an increase in the leakage current between the source and the drain that has made it impossible to obtain a large current amplification ratio (on/off ratio).
An object of the invention is to provide an organic thin-film transistor in which channel length is precisely controlled to improve basic transistor characteristics, and a method of manufacturing the transistor.
Another object of the invention is to provide a thin-film transistor that reduces leakage current arising between the source and the drain in cases in which the channel length is shortened.